Gong-Jian Tang

Pliocene-Quaternary crustal melting in central and northern Tibet and insights into crustal flow

There is considerable controversy over the nature of geophysically recognized low-velocity–high-conductivity zones (LV–HCZs) within the Tibetan crust, and their role in models for the development of the Tibetan Plateau. Here we report petrological and geochemical data on magmas erupted 4.7–0.3 Myr ago in central and northern Tibet, demonstrating that they were generated by partial melting of crustal rocks at temperatures of 700–1,050 °C and pressures of 0.5–1.5 GPa. Thus Pliocene-Quaternary melting of crustal rocks occurred at depths of 15–50 km in areas where the LV–HCZs have been recognized. This provides new petrological evidence that the LV–HCZs are sources of partial melt. It is inferred that crustal melting played a key role in triggering crustal weakening and outward crustal flow in the expansion of the Tibetan Plateau.

Genesis of pristine adakitic magmas by lower crustal melting: A perspective from amphibole composition

Genesis of adakites in arc and nonarc tectonic settings plays an important role in magmatic processes and material recycling along convergent margins. However, little is known about characteristics of pristine adakitic melts due to late stage magma evolutionary processes that dilute or obscure primary melt features, and thus, the genesis of adakites remains controversial. Here we present a detailed analysis of amphibole composition from Early Permian Awulale postcollisional adakitic diorite and granodiorite porphyries in the core of Tianshan Orogen, the central Asian orogenic belt. Two distinct populations of amphiboles, with markedly different aluminum contents, are observed in the adakitic rocks. These are (1) high-Al amphiboles, crystallizing as an early mineral phase at about 1 GPa, and (2) low-Al amphiboles, as a late mineral phase at 220–400 MPa, estimated on the basis of experimental phase equilibria data. Trace element modeling indicates that melts in equilibrium with the high-pressure amphiboles are of adakitic composition, suggesting that the Awulale pristine magmas already had an adakitic nature before the amphibole crystallization. This is consistent with the mafic lower crustal melting model, rather than a high-pressure basaltic melt fractionation, for adakite petrogenesis. Our results lend new insights into how amphibole composition can be used to constrain the geochemical characteristics of pristine adakitic magmas.